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I have an old telephone made in the sixties (FF-OB/ZB or FF53). Now, I was planning to make an external speaker for that telephone, but I failed miserably, so I've put that aside.

Now, the handset has a headphone, which accepts line levels and I had no problems connecting it to my sound mixer. The Microphone is a different story, though. I know it works correctly, as I can use the telephone without problems. However, when I connect it to the microphone leads (XLR connector) to the sound mixer, which has microphone amplification and phantom power (48V), the microphone appears to have a very weak pickup.

I need to turn up the gain almost to maximum, to get a signal out of it, and at that levels, it acquires huge amounts of static.

The microphone is a DKO48, here's a picture: enter image description here

I don't know, whether I should construct a preamp, or if I'm doing something fundamentally wrong. In case someone has experience with that sort of thing, please respond.

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  • \$\begingroup\$ I couldn't find any data for the microphone on a Google search, beyond one thing that suggested it was a dynamic microphone. How do you have the mixer input set up? High-Z or low? Balanced or unbalanced? \$\endgroup\$ – John R. Strohm Jan 28 '12 at 4:49
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IIRC almost all microphones used in telephones were carbon. If this is a carbon microphone (shake it and you should hear a noise of the carbon powder moving around) then you should connect it in series with a speaker and a DC power supply. That's it.

The microphone basically acts like a variable resistor.

If you want to use it to record something (that is, not just for connecting to a loudspeaker), you need to connect it in series with a resistor and a power supply, like this: schematic

To protect your equipment from DC offset, connect a small capacitor (10uF or so) in series with the "+" wire.

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  • \$\begingroup\$ OK, cool, I'm gonna rig this up on a breadboard real soon. Just one thing though: It says on a datasheet here, that the DKO48 is a "Transistor microphone capsule" I don't really know what that means, though. I checked the resistance and it's at 80k ohms and dropping (the longer the multimeter was connected, the lower the resistance read). \$\endgroup\$ – polemon Jan 28 '12 at 5:06
  • \$\begingroup\$ I just checked the voltage inside the handset and, it has a DC voltage of 2.8V, so I assume you were correct! I'll check in the morning when I've rigged up the parts on the breadboard. \$\endgroup\$ – polemon Jan 28 '12 at 5:16
  • \$\begingroup\$ @polemon If this is a "transistor microphone" then you should find out which pin is positive and which is negative. It should work in the circuit provided in my answer. \$\endgroup\$ – Pentium100 Jan 28 '12 at 6:00
  • \$\begingroup\$ Have my test setup rigged and it works as expected. \$\endgroup\$ – polemon Jan 28 '12 at 7:00

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